Silicon monocrystal is conventionally used as material for power semiconductor elements controlling high breakdown voltage and large current. At present, power semiconductor elements fall into several types, which are selectively used according to the intended use. For example, since bipolar transistors and insulated gate bipolar transistors (IGBTs) cannot be switched at high speed although current density can be increased, the use of bipolar transistors is limited up to several kHz and the use of IGBTs is limited up to about 20 kHz. On the other hand, power MOSFETs can be used at high speeds up to several MHz although large current cannot be handled.
However, since a power device supporting both large current and high-speed performance is strongly demanded by the market, particular efforts are made to improve IGBTs and power MOSFETs, which substantially reach the theoretical limit decided by materials.
FIG. 12 depicts a cross-sectional structure of a conventional MOSFET. An n− drift layer 302 is disposed on an n+ substrate (sub) 301; a p-base layer 303 is laminated on the n− drift layer 302; an n+ source layer 304 is selectively formed in a surface layer of the p-base layer 303; and a gate electrode 307 is formed on the n− drift layer 302 and the p-base layer 303 as well as the n+ source layer 304 via a source electrode 305 and a gate insulating film 306. Reference numeral 308 denotes a drain electrode.
Moreover, superjunction MOSFETs have recently gained attention. Superjunction MOSFETs are known as being published as a theory by Fujihira, et al in 1997 (see Non-Patent Literature 1) and being put into production as CoolMOSFET by Deboy, et al in 1998 (see Non-Patent Literature 2). These MOSFETs are characterized in that a P-layer is vertically formed into a columnar structure in an n− drift layer so as to drastically improve ON-resistance without deterioration in breakdown voltage characteristics between the source and drain.
Semiconductor materials are also studied in terms of power semiconductor elements and, as reported by Shenai (see Non-Patent Literature 3), SiC is recently attracting particular attention as a next generation power semiconductor element because the element is excellent in terms of low ON-voltage and high-speed/high-temperature properties. This is because SiC is, chemically, a very stable material with a wide band gap of 3 eV and can be used very stably as a semiconductor, even at high temperatures. Another reason for the recent attention is that SiC has a maximum field strength higher by one or more digits than silicon. SiC is very likely to exceed the material limit of silicon and is therefore largely expected to grow in use as power semiconductors or particularly MOSFETs in the future. SiC is particularly expected to achieve smaller ON-resistance, and it can be expected to realize a vertical SiC-MOSFET having lower ON-resistance with high breakdown voltage characteristics maintained.
A cross-sectional view of the structure of a typical SiC-MOSFET is the same as that in the case of silicon, as depicted in FIG. 12. The p-base layer 303 is laminated on the n− drift layer 302; the n+ source layer 304 is selectively formed on the surface layer of the p-base layer 303; the gate electrode 307 is formed on the n− drift layer 302 and the p-base layer 303 as well as on the n+ source layer 304 via the gate insulating film 306; and the drain electrode 308 is formed on the back surface of the substrate 301.
A SiC-MOSFET formed in this way is expected to be utilized as a switching device in the form of an element switchable at high speed with low ON-resistance in power conversion equipment such as an inverter for motor control and an uninterruptible power supply (UPS). SiC is a wide band gap semiconductor material and therefore, has a critical electric field strength that is about ten times higher than that of silicon as described above, and is expected to achieve sufficiently smaller ON-resistance; however, since the critical electric field strength of a semiconductor is increased by a factor of ten, a concentrated load of the electric field to an oxide film is also increased as compared to silicon elements particularly when a high voltage is applied. Therefore, the oxide film may be destroyed in the case of SiC consequent to a factor that does not cause a problem in a silicon power device because the critical electric field strength of silicon is reached before a larger electric field is applied to the oxide film. For example, a larger electric field is applied to the gate oxide film of the SiC-MOSFET depicted in FIG. 12, which may result in destruction of the gate oxide film or a significant problem in reliability. This may occur in not only SiC-MOSFETs but also SiC-IGBTs.
This problem will be described in detail with an example depicted FIG. 13 related to the structure of FIG. 12. FIG. 13 is a cross-sectional view of a unit cell thereof. This structure has a low-concentration n-type drift layer 202 deposited on a high-concentration n-type substrate 201, a high-concentration p-type gate layer 231 formed in a surface of the n-type drift layer 202 by ion implantation, and a low-concentration p-type layer 232 further deposited thereon. An n-type source layer 205 is selectively formed in a surface portion of the low-concentration p-type layer 232; a gate electrode 207 is formed via a gate oxide film 206 while a source electrode 209 is formed via an interlayer insulating film 208; and a channel region 211 is formed in the low-concentration p-type layer 232 immediately beneath the gate oxide film 206. An n-type base layer 204 penetrating the low-concentration p-type layer 232 to the n-type drift layer 202 is selectively formed as an counter layer by ion implantation of n-type impurities from the surface. Reference numeral 210 denotes a drain electrode.
Since the channel region 211 is formed in the low-concentration p-type layer without ion implantation, this structure can increase the mobility of conduction electrons and enables fabrication of a vertical MOSFET with a smaller ON-resistance. Since a vertical channel portion 224 of a well is completely blocked at a lower voltage by a depletion layer spreading laterally from the high-concentration p-type gate layer 231 to the low-concentration n-type drift layer 202 in a blocked state of the well, this structure is characterized in that leakage from an electric field to a gate oxide film, etc. near the channel region 211 can be prevented so as to increase the source/drain breakdown voltage. The blocked state of the well is achieved when the well is closed by the depletion layer.
However, even after the depletion layer spreading laterally from the high-concentration p-type gate layer 231 to the low-concentration n-type drift layer 202 has spread in the vertical channel portion 224 of the well in this structure, if the counter layer has a lower impurity concentration and a thinner thickness, a portion of the well is not closed by the depletion layer, allowing mobile electrons to reach near the interface with the gate oxide film 206 and apply a strong electric field to the gate oxide film 206 interposed between the gate electrode 207 and the n-type base layer 204, causing dielectric breakdown, which is a problem.
Non-patent Literature 1: Fujihira, et al, JJAP, Vol. 36, Part 1, No. 10, 1997, p. 6254.
Non-patent Literature 2: Deboy, et al, IEEE IEDEM 1998, p. 683
Non-Patent Literature 3: IEEE Transaction on Electron Devices, Vol. 36, 1989, p. 1811.